U.S. patent application number 11/897006 was filed with the patent office on 2008-03-27 for synchronising base stations.
This patent application is currently assigned to Ubiquisys Limited. Invention is credited to Edward Hatala, Peter Keevill.
Application Number | 20080075061 11/897006 |
Document ID | / |
Family ID | 37136975 |
Filed Date | 2008-03-27 |
United States Patent
Application |
20080075061 |
Kind Code |
A1 |
Hatala; Edward ; et
al. |
March 27, 2008 |
Synchronising base stations
Abstract
A method for synchronising a clock signal in a basestation of a
wireless telecommunications system is described. The basestation
has a reference clock signal and is operable to communicate with
wireless mobile terminals and with a packet switched network. The
method comprises detecting a radio frequency clock synchronisation
signal from a wireless telecommunications network, and
synchronising the reference clock signal of the basestation in
dependence upon the detected radio frequency clock synchronisation
signal.
Inventors: |
Hatala; Edward; (Devizes,
GB) ; Keevill; Peter; (Bath, GB) |
Correspondence
Address: |
BEYER WEAVER LLP
P.O. BOX 70250
OAKLAND
CA
94612-0250
US
|
Assignee: |
Ubiquisys Limited
|
Family ID: |
37136975 |
Appl. No.: |
11/897006 |
Filed: |
August 27, 2007 |
Current U.S.
Class: |
370/350 |
Current CPC
Class: |
H04W 56/0035 20130101;
H04B 7/2693 20130101; H04J 3/0664 20130101 |
Class at
Publication: |
370/350 |
International
Class: |
H04J 3/06 20060101
H04J003/06 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 29, 2006 |
GB |
0617004.7 |
Claims
1. A basestation for use in a wireless telecommunications system,
the basestation having a reference clock signal and being operable
to communicate with wireless mobile terminals and with a packet
switched network, and comprising: a detector for detecting a radio
frequency clock synchronisation signal from a wireless
telecommunications network; and a synchronisation unit operable to
synchronise the reference clock signal of the basestation in
dependence upon a detected radio frequency clock synchronisation
signal, wherein the synchronisation unit is further operable to
receive clock synchronisation data packets from a packet switched
network, and to synchronise the reference clock signal in
dependence upon received clock synchronisation data packets.
2. A basestation as claimed in claim 1, wherein the synchronisation
unit is operable to synchronise the reference clock signal using
the detected radio frequency clock synchronisation signal from a
macrocell basestation, or using received clock synchronisation data
packets if no macrocell basestation synchronisation signal can be
detected.
3. A basestation as claimed in claim 1, wherein the synchronisation
unit is operable to: receive a start data packet at the
basestation; receive a stop data packet at the basestation; obtain
timestamp information at the basestation, the information including
a time indication of time period between the sending of the start
data packet and the sending of the stop data packet; and measure a
time between receipt of the start data packet and the stop data
packet at the basestation using a reference clock signal of the
basestation; compare the timestamp information with the measured
time to produce a comparison value; and adjust the reference clock
signal of the basestation in dependence upon the generated
comparison value.
4. A method of operation of a basestation in a wireless
telecommunications system, the basestation generating a reference
clock signal, the method comprising: determining whether it is
possible to detect a radio frequency clock synchronisation signal
from a macrocell basestation of a wireless telecommunications
network; and, if so: synchronising the reference clock signal of
the basestation in dependence upon the detected radio frequency
clock synchronisation signal, or, if not: receiving clock
synchronisation data packets from a packet switched network; and
synchronising the reference clock signal in dependence upon
received clock synchronisation data packets.
5. A method as claimed in claim 4, wherein the step of receiving
clock synchronisation data packets from the packet switched network
comprises: receiving a start data packet at the basestation;
receiving a stop data packet at the basestation; and obtaining
timestamp information at the basestation, the timestamp information
including a time indication of time period between the sending of
the start data packet and the sending of the stop data packet; and
the step of synchronising the reference clock signal comprises:
measuring a time between receipt of the start data packet and the
stop data packet at the basestation using the reference clock
signal of the basestation, comparing the timestamp information with
the measured time to produce a comparison value; and adjusting the
reference clock signal of the basestation in dependence upon the
comparison value.
6. A basestation, for use in a wireless telecommunications system,
the basestation being adapted to: generate a reference clock
signal; receive a start data packet from a time server over a
packet data network; receive a stop data packet from the time
server over the packet data network; obtain timestamp information,
the timestamp information including a time indication of time
period between the sending of the start data packet and the sending
of the stop data packet; measure a time between receipt of the
start data packet and the stop data packet at the basestation using
the reference clock signal; compare the timestamp information with
the measured time to produce a comparison value; and adjust the
reference clock signal in dependence upon the generated comparison
value.
7. A method for synchronising a clock signal in a basestation of a
wireless telecommunications system, the basestation having an
internal reference clock signal and being operable to communicate
with wireless mobile terminals and with a packet switched network,
the method comprising detecting a radio frequency clock
synchronisation signal from a wireless telecommunications network,
and synchronising the reference clock signal of the basestation in
dependence upon the detected radio frequency clock synchronisation
signal.
8. A method as claimed in claim 7, comprising configuring at least
part of the basestation as a mobile terminal operable to obtain
clock synchronisation information from a standard wireless
telecommunications network.
9. A method as claimed in claim 8, comprising reconfiguring said
part of the basestation as a wireless basestation following
detection of the radio frequency clock synchronisation signal.
10. A method as claimed in any of claims 7 to 9, wherein the radio
frequency clock synchronisation signal is detected from a wireless
network different to the wireless network to which the basestation
is connected.
11. A method as claimed in claim 7, comprising receiving clock
synchronisation data packets from the packet switched network, and
synchronising the reference clock signal using the radio frequency
clock synchronisation signal in combination with the received clock
synchronisation data packets.
12. A method as claimed in claim 11, comprising initially
synchronising the reference clock signal using the detected radio
frequency clock synchronisation signal, and subsequently using
received clock synchronisation data packets to maintain reference
clock synchronisation, wherein a radio frequency clock
synchronisation signal is detected and used for synchronisation if
receipt of the clock synchronisation data packets is
interrupted.
13. A method as claimed in claim 11, wherein synchronising the
reference clock frequency using received clock synchronisation
packets includes the steps of: receiving a start data packet at the
basestation; starting a timer upon receipt of the start data
packet; receiving a stop data packet at the basestation; stopping
the timer upon receipt of the stop data packet to generate a timer
value; obtaining timestamp information at the basestation, the
information including a time indication of time period between the
sending of the start data packet and the sending of the stop data
packet; and comparing the timestamp information with the timer
value to produce a comparison value; and adjusting the reference
frequency of the base station in dependence upon the generated
comparison value.
14. A method as claimed in claim 13, wherein the timestamp
information is derived from information included in the start data
packet and the stop data packet.
15. A method as claimed in claim 13, wherein the timestamp
information is included in a timestamp data packet received by the
basestation.
16. A method as claimed in claim 11, wherein the synchronising the
reference clock signal does not require transmission of feedback
data packets from the basestation to the packet switched
network.
17. A method as claimed in any of claims 7 to 9, wherein the packet
switched network is an Internet protocol based network.
18. A basestation for use in a wireless telecommunications system,
the basestation having a reference clock signal and being operable
to communicate with wireless mobile terminals and with a packet
switched network, and comprising: a detector for detecting a radio
frequency clock synchronisation signal from a wireless
telecommunications network; and a synchronisation unit operable to
synchronise the reference clock signal of the basestation in
dependence upon a detected radio frequency clock synchronisation
signal.
19. A basestation as claimed in claim 18, comprising
synchronisation unit is operable to configure at least part of the
basestation as a mobile terminal operable to obtain clock
synchronisation information from a standard wireless
telecommunications network.
20. A basestation as claimed in claim 19, wherein the
synchronisation unit is operable to reconfigure said part of the
basestation as a wireless basestation following detection of the
radio frequency clock synchronisation signal.
21. A basestation as claimed in any one of claims 18 to 20, wherein
the radio frequency clock synchronisation signal is detected from a
wireless network different to the wireless network to which the
basestation is connected.
22. A basestation as claimed in claim 18, wherein the
synchronisation unit is operable to receive clock synchronisation
data packets from a packet switched network, and to synchronise the
reference clock signal using a radio frequency clock
synchronisation signal in combination with received clock
synchronisation data packets.
23. A basestation as claimed in claim 22, wherein the
synchronisation unit is operable to synchronise the reference clock
signal initially using the detected radio frequency clock
synchronisation signal, and subsequently using received clock
synchronisation data packets to maintain reference clock
synchronisation, and to detect and use a radio frequency clock
synchronisation signal for synchronisation if receipt of the clock
synchronisation data packets is interrupted.
24. A basestation as claimed in claim 22, wherein the
synchronisation unit is operable to: receive a start data packet at
the basestation; start a timer upon receipt of the start data
packet; receive a stop data packet at the basestation; stop the
timer upon receipt of the stop data packet to generate a timer
value; obtain timestamp information at the basestation, the
information including a time indication of time period between the
sending of the start data packet and the sending of the stop data
packet; and compare the timestamp information with the timer value
to produce a comparison value; and adjust the reference clock
signal of the basestation in dependence upon the generated
comparison value.
25. A basestation as claimed in claim 24, wherein the timestamp
information is derived from information included in the start data
packet and the stop data packet.
26. A basestation as claimed in claim 24, wherein the timestamp
information is included in a timestamp data packet received by the
basestation.
27. A basestation as claimed in any one of claims 22, wherein the
basestation is not operable to transmit feedback data packets to
the packet switched network following reception of clock
synchronisation data packets therefrom.
28. A basestation as claimed in any one of claims 18 to 20, wherein
the packet switched network is an Internet protocol based
network
29. A method for synchronising a clock signal in a basestation of a
wireless telecommunications system, the basestation having an
internal reference clock signal, and being operable to communicate
with wireless mobile terminals and with a packet switched network,
the method comprising receiving clock synchronisation data packets
transmitted from the packet switched network, and synchronising the
reference clock signal of the basestation in dependence upon the
received clock synchronisation packets, wherein the method does not
require transmission of feedback data packets from the basestation
to the packet switched network.
30. A method as claimed in claim 29, wherein the packet switched
network is an Internet protocol based network.
31. A method as claimed in claim 29, comprising the steps of:
receiving the start data packet at the basestation; starting a
timer upon receipt of the start data packet; receiving the stop
data packet at the basestation; stopping the timer upon receipt of
the stop data packet to generate a timer value; obtaining timestamp
information at the basestation, the information including a time
indication of time period between the sending of the start data
packet and the sending of the stop data packet; and comparing the
timestamp information with the timer value to produce a comparison
value; and adjusting the reference frequency of the base station in
dependence upon the generated comparison value.
32. A method as claimed in claim 31, wherein the timestamp
information is derived from information included in the start data
packet and the stop data packet.
33. A method as claimed in claim 31, wherein the timestamp
information is included in a timestamp data packet received by the
basestation.
34. A method as claimed in claim 29, wherein the timestamp
information is derived from information included in the start data
packet and the stop data packet.
35. A method as claimed in claim 29, wherein the timestamp
information is included in a timestamp data packet received by the
basestation.
36. A basestation for use in a wireless telecommunications system,
the basestation having an internal reference clock signal, and
being operable to communicate with wireless mobile terminals and
with a packet switched network, and comprising a synchronisation
unit operable to receive clock synchronisation data packets
transmitted from the packet switched network, and to synchronise
the reference clock signal of the basestation in dependence upon
received clock synchronisation packets, synchronising the reference
clock signal does not require transmission of feedback data packets
to the packet switched network.
37. A basestation as claimed in claim 36, wherein the packet
switched network is an Internet protocol based network.
38. A basestation as claimed in claim 36 or 37, wherein the
synchronisation unit is operable to: receive a start data packet at
the basestation; start a timer upon receipt of the start data
packet; receive a stop data packet at the basestation; stop the
timer upon receipt of the stop data packet to generate a timer
value; obtain timestamp information at the basestation, the
information including a time indication of time period between the
sending of the start data packet and the sending of the stop data
packet; and compare the timestamp information with the timer value
to produce a comparison value; and adjust the reference clock
signal of the base station in dependence upon the generated
comparison value.
Description
[0001] The present invention relates to synchronising base stations
in wireless telecommunications systems.
BACKGROUND OF THE INVENTION
[0002] Wireless telecommunications systems make use of basestations
which communicate with mobile terminals using a radio frequency air
interface. Such systems typically have many basestations
communicating with many more mobile terminals. In order to connect
communications from a mobile terminal to another user, the
basestations communicate with an operator's network, typically
using a circuit switched network. This network is often known as a
"backhaul" network.
[0003] In order that mobile terminals and basestations are able to
communicate with one another, and for one basestation to handover
communication with a mobile terminal to another basestation, it is
important that internal clock signals in the basestations are
synchronised with one another and with the network.
[0004] Existing basestations use expensive oven controlled crystal
oscillators to maintain a stable reference clock signal. However,
over time the reference clock signal will drift from its nominal
value, resulting in service deterioration, such as a mobile
terminal not being able to connect to the network. To counteract
the drift in reference clock signal, synchronisation signals are
sent from a highly accurate and stable master clock source in the
network. In existing time division multiplexed (TDM) backhaul
networks, this clock is distributed to the basestations as embedded
pulses at the physical "wire" level. These synchronisation pulses
are used to "pull-in" or keep the basestation crystal oscillator
within specification. In the US, some networks basestations are
synchronised to signals derived from GPS (Global Positioning
System) signals.
[0005] FIG. 1 of the accompanying drawings shows synchronisation
boundaries of a circuit switched network 1 which includes a core
network top-level clock 10. This top-level clock sets the timing
reference for the whole system, and its output is transferred to
the network via a Primary Reference Clock (PRC) 12. In this
example, a synchronous digital hierarchy (SDH) network 14 is used
to transfer data between the core network and a radio network
controller (RNC) 16, as is well known. The RNC 16 communicates with
basestations 20a, 20b, and 20c via a Plesiochronous Digital
Hierarchy (PDH) network 18. Leased lines can provide the PDH
network 18, for example.
[0006] A recent development in the provision of mobile radio
networks provides residential basestations, which are smaller and
lower cost than existing large scale designs, for communicating
with wireless mobile terminals. Such residential basestations make
use of existing broadband fixed line connections as the backhaul
network. Such broadband networks are typically provided by Internet
Protocol (IP) based networks, for example an ADSL (Asynchronous
Digital Subscriber Line) network.
[0007] However, such IP based networks typically do not make use of
timing synchronisation signals. This means that the reference clock
signal of the residential is likely to drift out of frequency
specification, especially because residential basestations
generally will make use of a less highly specified crystal
oscillator than that used for the usual wireless network
basestation.
[0008] Existing data applications running over IP networks do not
require nor provide synchronisation pulses, as the layer 2 protocol
encapsulating IP performed this task. In the case of an ADSL
backhaul network the Point-to-Point Protocol (PPP) layer 2 protocol
is used to frame IP data. In turn, PPP is framed by the
Asynchronous Transfer Mode (ATM) Adaptation Layer 5 (ML5), for
transport over the ATM network between the ISP and ADSL modem PPP
termination point in the home. Whilst the ADSL modems have
reference clock signals which are adjusted to synchronise to the
incoming ADSL physical level signals, these do not provide
sufficient precision for use as a reference clock signal for a
wireless basestation.
[0009] Although some mechanisms exist to provide time
synchronisation over an IP network, for example as Network Time
Protocol (NTP), none exist that provide sufficient accuracy for a
basestation to attain and remain within frequency specification
suitable for the wireless communications system. Furthermore, the
methods that do exist rely on a return signalling path from the
remote equipment to the NCS--a residential basestation deployment
of thousands or millions of devices could create overwhelming
amounts of return traffic to the NCS. Such large amounts of return
data would limit the scalability of such a system. In particular,
in ADSL connections the return path to the network is often of
significantly lower bandwidth than the forward path, and so
unnecessary traffic on the return link is particularly
undesirable.
[0010] Drift in the reference clock signal can also occur with
changes in temperature. Again, whilst methods for compensating for
reference clock signal drift with temperature compensation exist,
the degree of accuracy for the price is prohibitive for the
residential market.
[0011] GSM and W-CDMA cellular network Base Stations are required
to operate at the same frequency (within a given tolerance),
although a delay between clock signals can be tolerated.
SUMMARY OF THE PRESENT INVENTION
[0012] According to one aspect of the present invention, there is
provided a method for synchronising a clock signal in a basestation
of a wireless telecommunications system, the basestation having an
internal reference clock signal and being operable to communicate
with wireless mobile terminals and with a packet switched network,
the method comprising detecting a radio frequency clock
synchronisation signal from a wireless telecommunications network,
and synchronising the reference clock signal of the basestation in
dependence upon the detected radio frequency clock synchronisation
signal.
[0013] Such a method can also comprise configuring at least part of
the basestation as a mobile terminal operable to obtain clock
synchronisation information from a standard wireless
telecommunications network. In such a case, the method can comprise
reconfiguring said part of the basestation as a wireless
basestation following detection of the radio frequency clock
synchronisation signal.
[0014] The radio frequency clock synchronisation signal may be
detected from a wireless network different to the wireless network
to which the basestation is connected.
[0015] The method may comprises receiving clock synchronisation
data packets from the packet switched network, and synchronising
the reference clock signal using the radio frequency clock
synchronisation signal in combination with the received clock
synchronisation data packets.
[0016] In such a case, the method may comprise initially
synchronising the reference clock signal using the detected radio
frequency clock synchronisation signal, and subsequently using
received clock synchronisation data packets to maintain reference
clock synchronisation, wherein a radio frequency clock
synchronisation signal is detected and used for synchronisation if
receipt of the clock synchronisation data packets is
interrupted.
[0017] In a method in which synchronisation packets are received,
then synchronising the reference clock frequency using received
clock synchronisation packets may include the steps of receiving a
start data packet at the basestation, starting a timer upon receipt
of the start data packet receiving a stop data packet at the
basestation, stopping the timer upon receipt of the stop data
packet to generate a timer value, obtaining timestamp information
at the basestation, the information including a time indication of
time period between the sending of the start data packet and the
sending of the stop data packet, comparing the timestamp
information with the timer value to produce a comparison value, and
adjusting the reference frequency of the basestation in dependence
upon the generated comparison value.
[0018] Preferably, the basestation does not transmit feedback data
packets to the packet switched network following reception of clock
synchronisation data packets therefrom.
[0019] The packet switched network may be an Internet protocol
based network.
[0020] According to another aspect of the present invention, there
is provided a basestation for use in a wireless telecommunications
system, the basestation having a reference clock signal and being
operable to communicate with wireless mobile terminals and with a
packet switched network, and comprising: [0021] a detector for
detecting a radio frequency clock synchronisation signal from a
wireless telecommunications network; and [0022] a synchronisation
unit operable to synchronise the reference clock signal of the
basestation in dependence upon a detected radio frequency clock
synchronisation signal.
[0023] According to another aspect of the present invention, there
is provided a method for synchronising a clock signal in a
basestation of a wireless telecommunications system, the
basestation having a reference clock signal, and being operable to
communicate with wireless mobile terminals and with a packet
switched network, the method comprising receiving clock
synchronisation data packets transmitted from the packet switched
network, and synchronising the reference clock signal of the
basestation in dependence upon the received clock synchronisation
packets, wherein the method does not include transmitting feedback
data packets to the packet switched network from the basestation in
response to receipt of clock synchronisation data packets
therefrom.
[0024] According to another aspect of the present invention, there
is provided a basestation for use in a wireless telecommunications
system, the basestation having an internal reference clock signal,
and being operable to communicate with wireless mobile terminals
and with a packet switched network, and comprising a
synchronisation unit operable to receive clock synchronisation data
packets transmitted from the packet switched network, and to
synchronise the reference clock signal of the basestation in
dependence upon received clock synchronisation packets,
synchronising the reference clock signal does not require
transmission of feedback data packets to the packet switched
network.
[0025] A number of novel approaches will be described that deliver
the accuracy of synchronisation that is required for high frequency
wireless systems, such GSM/UMTS (Global System for Mobile
Communications/Universal Mobile Telecommunications System)
residential basestations. These approaches are also applicable for
other applications such as IP Television (IPTV).
[0026] The approaches described below with reference to embodiments
of the present invention, can be summarised as follows: [0027]
Deriving a timing reference from other wireless basestations [0028]
A method of providing very accurate timing over a packet network
(such as IP) using only the forward link from the network to the
basestation [0029] A method of compensating the timing reference
for network jitter [0030] A method of temperature compensation of
the crystal oscillator such that a low-cost crystal can be used
[0031] A method to switch between timing reference sources
(basestation, IP network) [0032] Minimise network synchronisation
loading by piggybacking on to other messages
BRIEF DESCRIPTION OF THE DRAWINGS
[0033] FIG. 1 illustrates a previously considered basestation clock
synchronisation technique;
[0034] FIG. 2 illustrates basestation clock synchronisation
according to one aspect of the present invention;
[0035] FIG. 3 illustrates signal transfer in a technique according
to the present invention; and
[0036] FIG. 4 illustrates a data packet structure according to
another aspect of the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0037] FIG. 2 illustrates a wireless telecommunications network
that uses the known backhaul network shown in FIG. 1, together with
a broadband IP based network. A residential basestation is shown at
44, and operates in parallel with other basestations 20a, 20b, and
20c. As before, the top-level clock 10 provides the basic timing
reference for the system. It will be readily appreciated that the
term "residential basestation" can include any appropriate
basestation suitable for use in a wireless telecommunications
network.
[0038] In the system illustrated in FIG. 2, and as is well known
and understood, data is communicated between the core network using
a gateway 31, and a router 34 connected to the internet 36. A
Broadband Remote Access Server (BRAS) 38 connects the Internet
connection with an ADSL network 40, which itself provides a
broadband connection for the residential basestation 44, via a
Digital Subscriber Line Access Multiplexer (DSLAM) 42.
[0039] In a first embodiment of the present invention, reference
clock signal synchronisation for the residential basestation 44 is
provided by reconfiguring basestation modem and RF functions to
operate such that they function as a GSM/UMTS terminal device in
order to recover timing synchronisation from surrounding
basestations in the manner that GSM/UMTS mobile terminals achieve
this. This reconfiguration is of a short duration--typically a few
minutes--and the recovered timing synchronisation is used to
"discipline" a crystal oscillator which retains the recovered
timing accuracy for a number of hours. The modem and RF functions
are then reconfigured as a basestation to provide service to mobile
terminals, for example for subscribers within the home. This
pattern of reconfiguration can be repeated to maintain accurate
timing for basestation service. Intelligence is available within
the basestation control software to prevent service impacts to
users so the reconfiguration will only occur when there are no
active calls and/or at likely quiet periods during the day
determined by long-term observation of call patterns.
[0040] The timing synchronisation signals can be detected from any
available GSM/UMTS transmission, and not just from the network to
which the residential basestation is connected. Similarly, timing
derived from GSM networks can be used to define UMTS basestation
timing and vice versa.
[0041] In some deployment scenarios, it is not possible to derive a
suitable timing synchronisation signal. For example, surrounding
basestation signals may be too weak to be useable. Also, it may not
be desirable to repeatedly configure the basestation as a mobile
terminal for detecting the timing signal, since the configuration
and reconfiguration of the basestation takes time.
[0042] In those cases, an alternative method for synchronisation is
provided in which the IP network broadband connection is used to
supply a timing reference signal.
[0043] Using IP as a timing reference source presents a new
challenge, namely jitter, which can severely limit the feasibility
of using the received data packets to provide the required level of
accuracy. This challenge can be overcome according to another
aspect of the present invention, by accurately timestamping a first
(or start) data packet indicating when the data packet was sent
from the core network to the basestation. This timestamp is sent
over the IP layer, using a network clock server (NCS) 32.
[0044] In order to calibrate the basestation clock frequency, the
basestation uses a timer or counter which is started upon reception
of a start data packet, and is stopped upon reception of a stop
data packet.
[0045] The basestation local clock runs asynchronously to that of
the network clock, that is, excluding errors due to jitter, the
basestation may count more or less time between the network start
and stop data packets than the difference between the received
start and stop timestamps which are generated by the network
clock.
[0046] Using a radio network frequency synchronisation pulse from a
surrounding basestation, the residential basestation clock can be
synchronised, resulting in the ability to measure the jitter
between the network start and stop packets with an accuracy of, for
example, 50 ppb. The basestation counter can then be calibrated by
calculating the difference between the counted time difference, and
the time difference indicated by the time stamped start and stop
data packets. This difference is the jitter time, which can be used
to accurately compensate for jitter, thereby accurately
synchronising to the NCS clock timestamp.
[0047] If the residential basestation cannot detect a macro cell
basestation synchronisation signal then it proceeds to use a second
method of synchronisation. This method is to measure the amount of
clock pulses over a given time period; longer measurement periods
will result in greater accuracy. As start/stop packets may be lost,
the residential basestation can decide when to start the long-term
timer/counter of its clock. If the residential basestation had
previously been synchronised to a wireless basestation
synchronisation signal then the residential basestation can decide
to synchronise using a short time between a single start-stop data
packet duplets.
[0048] The NCS 32 sends start and stop packet continuously, the
interval between these packets can be adjusted to manage the
network loading created by the packets and residential basestation
accuracy required. While the residential basestation is integrating
its clock between a given NCS start packet (for example packet 1)
and stop packet (for example packet 1000), the time between each
start and stop packet pair is measured and the jitter time noted.
Over the long-term integration period, the jitter between the
start-stop pairs will be distributed in time, between the least
delayed (minimum jitter) and longest delayed (maximum jitter).
After the long-term integration is completed, the amount of time
correction needed to compensate for the jitter can be
calculated.
[0049] The method of using the least amount of jitter time for
residential basestation timing synchronisation means that the
propagation delay from the NCS 32 to the basestation 44 is not
required as is the case in many other synchronisation processes.
Over time, the minimum jitter delay will approximate to propagation
delay.
[0050] Apart from jitter, the residential basestation clock
reference frequency can vary with temperature. In embodiments of
the present invention, the amount of clock frequency deviation from
nominal per degree centigrade is characterised. This information is
stored in memory in the basestation so that it can be accessed to
enable the frequency deviation due to temperature to be
compensated. A temperature measurement device is included in the
basestation. Periodically, the temperature is measured and the
temperature is used to index the frequency deviation stored in
memory. The deviation value is then used to adjust the crystal
oscillator either directly or by a compensation factor applied to
the start packet/stop pack timing measurements which will
indirectly adjust the oscillator.
[0051] Once the crystal oscillator has been compensated for
temperature, and the long term integration of start and stop
packets used to compensate for jitter, the residential basestation
clock can be synchronised with the network clock. Synchronisation
can then be achieved by comparing the time between the start and
stop data packets received from the network and the time counted by
the basestation timer/counter. Any difference will initiate
appropriate adjustment of the basestation's crystal oscillator to
obtain synchronisation to the network clock. Note that in this
application only frequency accuracy is a concern--the basestation
does not have to be fully phase synchronous with the network
clock.
[0052] As synchronisation packets can increase the packet loading
on the network, a method is proposed to reduce this loading in
which synchronisation messages (timestamps) are transmitted within
other messages such as "keep-alive" messages.
[0053] FIG. 3 illustrates packet transfer from the NCS 32 to the
residential basestation 44. The arrowed lines 52 illustrate the
timing of packets being sent 50 at times t0, t1 etc from the NCS 32
to the home basestation 44. The packets are shown as being sent on
a regular time boundary, but this need not be the case. The time
period taken for the time the sloped lines in FIG. 3 illustrate
stamped data packets. The steeper the angle of the slope, the
greater the time delay. If the delay in the network were constant
(as illustrated by the dotted line in FIG. 3) then the angles of
the lines would be also be constant. This constant time delay would
then represent the propagation delay of a packet transmitted from
the NCS 32 to the basestation 44. Network jitter can cause the
angle to vary on below that of the constant angle. It will also
cause it to vary from one packet to another.
[0054] In order to derive the minimum packet arrival time, the
following packet sequence is required: a start packet is sent from
the NCS 32 to start the residential basestation timer counter, and
a predefined time later, a stop packet is sent from the NCS 32. The
stop packet causes the basestation timer counter to be stopped, and
contains a timestamp of when the corresponding start packet was
sent. Finally, a third packet is sent by the NCS 32 which contains
a timestamp of when the stop packet was sent. The start and stop
packets may each experience jitter, and, therefore, the time
difference between the start-stop of the basestation timer counter
and the difference between the start and stop timestamps from the
NCS, represents the jitter time, inclusive of the start and stop
packets. This jitter time is divided by two.
[0055] Although the result may be greater than the actual jitter of
a single packet, over time both packets will experience minimum
jitter. The method then determines the minimum jitter by comparing
a new jitter value with the pervious jitter value, and if it is
less then uses the new jitter value as the reference jitter value.
The jitter discovery packets (start, stop and timestamp packets)
are sent regularly, so that jitter discovery is a continuous
process. This is especially useful when the characteristics of the
network change over time.
[0056] The jitter discovery packets from the NCS are used by the
basestation to initiate start and stop long-term measurement of the
basestation clock. For example, the basestation decides to use
start jitter discovery packet "x" it stores the associated NCS
timestamp in memory and reads the basestation timer counter which
it also stores in memory. At a pre-determined jitter packet count
or time-out the basestation reads the next stop jitter discovery
packet "x+n" where n is the number of packets from the first. At
the same time as reading the stop jitter discovery packet NCS
timestamp and storing it in memory the basestation reads its timer
counter whose value is also stored.
[0057] The basestation then determines the time between its timer
counter start and stop time which is stored in memory. The next
step is to determine the time difference between the network NCS
timestamp and the time between the basestation start and stop timer
counter. The longer the time between starting (read) and stopping
(read) of the basestation timer counter the better the accuracy.
With a relatively large number of time delay measurements, it is
possible to analyse the distribution of the jitter values, to
enable jitter to be effectively compensated for. Information
regarding the jitter statistics can also be used to optimise the
packet buffer size, and hence to minimise latency in the packet
based system.
[0058] Either the Basestation or the NCS can initiate the
synchronisation process. If it is the NCS then an extra message set
is defined whereas the above those defined for establishing the
current minimum jitter is required, to start and stop the
process.
[0059] The synchronisation process consists starting the
basestation timer counter and then after a period of time stopping
the timer counter. The time when the clock was started and stopped
is derived from the NCS timestamp. The difference between this time
and the time counted using the basestation clock is the
synchronisation error which also includes jitter errors. Errors due
to temperature are compensated for at each jitter measurement
process.
[0060] If the temperature of the basestation shifts from a nominal
value, then the reference frequency of the basestation's crystal
oscillator will drift from its nominal value. In embodiments of the
present invention, frequency errors for the crystal oscillator are
measured over a range of temperatures. The temperatures and
corresponding errors are stored in memory in the basestation.
[0061] A temperature-measuring device is incorporated in the
basestation and when the temperature drifts outside of a given
range, then the following temperature compensation process is
initiated. This process consists of reading the temperature from
the device and using that temperature to index the crystal
oscillator error value stored in memory. This value is then used to
adjust the crystal oscillator in order to compensate the output
frequency for the temperature change.
* * * * *